1. Field of the Invention
The present invention generally relates to a microwave heating apparatus. More specifically, the present invention relates to a microwave heating apparatus adapted for uniformly heating a sheet-like material such as a paper sheet and the surfaces of other types of a material being heated.
2. Description of the Prior Art
Conventionally it has been well-known that a material to be heated may be heated by a microwave. However, in heating a material having a large surface area as compared with the volume thereof, such as a paper sheet, by the use of a conventional microwave heating apparatus, the heating efficiency is decreased and hence the electric field strength of the microwave need be increased. Accordingly, it was not easily possible to effectively and uniformly heat a sheet-like material, such as a paper sheet, by the use of a conventional microwave heating apparatus.
Of late, a microwave heating apparatus adapted for heating a sheet-like material being heated using a rectangular waveguide has been proposed and is disclosed in Japanese Patent Publication No. 873/1980, for example. FIG. 1 is a perspective view showing a portion of one example of a conventional microwave heating apparatus which constitutes the background of the invention. The conventional microwave heating apparatus shown comprises a rectangular waveguide 1, having leakage openings 2 formed on the top surface thereof. When microwaves of 2450 MHz, for example, are supplied to the rectangular waveguide 1, the microwaves leak through the openings 2. Therefore, a sheet-like material being heated, such as a paper sheet, being transferred close to the waveguide 1 is heated with the leaked microwaves. According to this conventional approach, it is possible to uniformly heat a sheet-like material or the surface of a material being heated having a given thickness.
However, the FIG. 1 conventional approach still involves the shortcomings to be solved set forth in the following. More specifically, the size of a rectangular waveguide shown in FIG. 1 depends on the cut-off frequency and mode and, assuming that the frequency used is selected to be 2450 MHz, as described above, the internal geometry must be 109.2 mm.times.54.6 mm. Accordingly, a microwave heating apparatus employing a rectangular waveguide as shown in FIG. 1 has a large volume. On the other hand, it is also well-known that a microwave heating apparatus can be utilized to fuse a toner in an electrophotographic machine. However, in the light of a recent demand for a small sized or compact electrophotographic machines, incorporation of such microwave heating apparatus employing a rectangular waveguide having a large volume as described above in such electrophotographic machine makes it impossible to meet such demand for small sized or compact implementation.